All vertebrates have thyroid glands. It is not essential for life but it is necessary for growth and mental well being

1. Thyroid and Antithyroidal agents

2.  Hormones are classified generally 1- thyroid hormones 2-
parathyroid hormones 3- pituitary hormones 4- Pancreatic
hormones 5- Adrenal gland hormones 6- Sex hormones
 Thyroid hormones –
 All vertebrates have thyroid glands.
 It is not essential for life but it is necessary for growth and mental well being.
 In a healthy adult, it weight about 25 g and is composed of two lobes which lie
one on either side of the larynx.
 It is longer in women then in men.
 At the posterior surface of each thyroid gland, can be seen the parathyroid
gland.
 The hormone has molecular weight of about 680000 and it contains organically
bound iodine.
 A large quantity of iodine (0.06%) is present in thyroid gland, almost all of which
is firmly bound to protein largely in the form of the thyroid hormones.

3.  On hydrolysis , thyroglobulin is converted to several iodinated
tyroine derivatives.
 Thyroid hormones are part of thyroglobulin molecule which is
formed in the microsomes of the follicular cells.
 Thyroxine and tri-iodotyrosine are the principal thyroid hormones.
 Chemically they are amino acid containing iodinated diphenyl
ethers.
 tri-iodotyrosine is more active then Thyroxine
 These hormones necessary for
 1- oxygen consumption, heat production and metabolism of carbohydrate, fats
and proteins
 2- proper functioning of gastrointestinal tract, cardiovascular, reproductive,
skeletal and neuromuscular system.
 3- optimal functioning of catecholamine, antidiuretic hormones.
 4- normal growth and differentiation.

5. Biosynthesis , storage and metabolism of thyroid
hormones.
 The biosynthesis of thyroid hormones is regulated by variations in
the plasma levels of thyroid stimulating hormones (TSH) from the
anterior pituitary gland.
 The conversion of inorganic iodide to thyroid hormones involves
following steps-
 1- iodine uptake by the gland
 2- oxidation of iodine and iodination of tyrosyl groups
 3- formation of thyroxine and tri-iodothyronine from iodotyrosines
 4- release of thyroxine and tri-iodothyronine.

7. SAR
 1- the two phenyl rings must be connected by an ether, thioether
or methylene linkage in order to maintain the activity.
 2- a carboxyl group must be present in aliphatic side chain.
 3- halogen and methylene substituent's may be placed at 3 and 5
position.
 4- the 3 monosubstituted analogs are more active than the 3, 5
disubstituted compounds.
 5- for maximal activity, position 4 should be occupied by a hydroxyl
group
 6- the bulky and lipophilic group at 3 position enhance the activity.

8. Antithyroid drugs
 These drug act by inhibition of the synthesis of thyroid hormones.
 They exert immediate effects since they act at the first stage of
iodine incorporation by the gland.
 They are categorized as under
 1) Thiomides
 2) Aniline derivative
 3) polyhydric phenols
 4) ionic inhibitors that block the uptake of iodine by the gland
 5) Miscellaneous agents.
 1) Thiomides- thiourea and thiouracil derivative are among the
primary drugs to treat thyroid hyperactivity.

9.  The methyl and propylthiouracil are effective drug. Skin rashes are
the most common side effects of these drugs.

10. 2)Aniline derivative-
 These agents interfere with some of the processes catalyzed by thyroid peroxidase
like, iodide oxidation, organification and coupling of iodotyrosines.

11.  3) Polyhydric Phenols-
 The only clinical agents from this category is resorcinol.
 4) Ionic inhibitors-
 These anions resemble iodide ions and affects the power of thyroid gland to
accumulate iodide.
 e.g. Potassium perchlorate, thiocyanate.
 5) miscellaneous agents-
 Lithium carbonate- lithium appears to prevent the release of both hormonal
and non-hormonal iodine from the thyroid gland.